I don't see why you couldn't use the duck fat to roast the potatoes. But you might want olive oil or another liquid fat for the dressing. I think the dressing would be too thick if made with duck fat. Of course, you could melt the fat and then make the dressing and pour it over the potatoes while warm.

Assuming you mean for roasting the potatoes - duck fat sounds wonderful. Never encountered duck fat Patatas Bravas in Spain, but would have been happy to! (Duck fat and potatoes are good friends in general.)